Skeletal development in sloths and the evolution of mammalian vertebral patterning

Lionel Hautier, Vera Weisbecker, Marcelo R. Sánchez-Villagra, Anjali Goswami, Robert J. Asher

Research output: Contribution to journalArticlepeer-review

95 Citations (Scopus)


Mammals show a very low level of variation in vertebral count, particularly in the neck. Phenotypes exhibited at various stages during the development of the axial skeleton may play a key role in testing mechanisms recently proposed to explain this conservatism. Here, we provide osteogenetic data that identify developmental criteria with which to recognize cervical vs. noncervical vertebrae in mammals. Except for sloths, all mammals show the late ossification of the caudal-most centra in the neck after other centra and neural arches. In sloths with 8-10 ribless neck vertebrae, the caudal- most neck centra ossify early, matching the pattern observed in cranial thoracic vertebrae of other mammals. Accordingly, we interpret the ribless neck vertebrae of three-toed sloths caudal to V7 as thoracic based on our developmental criterion. Applied to the unusual vertebral phenotype of long-necked sloths, these data support the interpretation that elements of the axial skeleton with origins from distinct mesodermal tissues have repatterned over the course of evolution.

Original languageEnglish
Pages (from-to)18903-18908
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number44
Publication statusPublished - 2 Nov 2010
Externally publishedYes


  • Constraint
  • Heterochrony
  • Ontogeny
  • Vertebrae
  • Xenarthra


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